1. Field
This disclosure relates to regulation of cell growth, and more particularly to regulation of cancer cell growth. In particular, peptides and polypeptides derived from particular regions of syndecan 1 have been shown to inhibit activation of α4β1 integrin (also known as very late antigen-4, VLA-4, and engagement of VLA-4 by vascular endothelial growth factor receptor-2 (VEGFR2), thereby limiting tissue invasion.
2. Related Art
Multiple myeloma, a disease in which malignant plasma cells form disruptive bone tumors, is the second most prevalent hematologic malignancy in the United States (Laubach et al., 2010). The emergence of new therapies (e.g., bortezomib, thalidomide) has greatly improved survival rates in patients with myeloma (Laubach et al., 2010). However, these therapies slow rather than cure the disease and patients develop resistance and become refractory over the course of treatment. Thus, the need for novel therapies that prevent the progression of the disease and maintain patient quality of life remains a high priority. A better understanding of the mechanisms involved in disease progression may identify new and effective targets for such therapies.
Heparanase (HPSE), an endo-β-D-glucuronidase that degrades heparan sulfate (HS) chains on proteoglycans, is a tumor promoter in multiple myeloma, as well as in many other cancers (Barash et al., 2010, Kelly et al., 2003 and Vlodavsky et al., 2002). HPSE cleaves at highly specific sites within HS chains, releasing biologically active fragments 5 to 7 kDa in size that bind and promote the activity of heparin-binding growth factors. However, HPSE has far-reaching effects beyond the release of HS fragments, including altering the expression of genes that affect the proliferation, invasion and survival of tumor cells and other cells in the tumor microenvironment (Vlodavsky et al., 2002 and Levy-Adam et al., 2010). A major target of HPSE in multiple myeloma is syndecan-1 (Sdc1, CD138), one of a family of cell surface heparan sulfate proteoglycans found on most cells. Sdc1 is highly expressed on malignant plasma cells and has a causal role in multiple myeloma (Khotskaya et al., 2009; O'Connell et al., 2004; Sanderson and Yang, 2008 and Yang et al., 2002). Cells expressing high levels of Sdc1 exhibit enhanced invasion into collagen gels in vitro and as tumors in vivo (Yang et al., 2002). In contrast, suppression of Sdc1 expression causes apoptosis in myeloma cells (Khotskaya et al., 2009 and Wu et al., 2012).
Induction of metalloproteinase-9 (MMP-9) expression by HPSE, along with its pruning of the HS chains on Sdc1, causes MMP-9-mediated shedding of Sdc1 ectodomain into the tumor microenvironment where the proteoglycan enhances angiogenesis and is likely to have roles in myeloma cell adhesion, proliferation and survival (Yang et al., 2007; Mahtouk et al., 2007; Purushothaman et al., 2008 and Purushothaman et al., 2010). (Ramani et al., JBC, 287: 9952-9961 (2012). Indeed, high levels of shed Sdc1 in serum correlate with poor prognosis in multiple myeloma (Seidel et al., 2000 and Scudla et al., 2010). Although Sdc1 is shed, the steady-state level of functional Sdc1 at the cell surface remains unchanged due to an HPSE-induced increase in receptor expression (Yang et al., 2007; Mahtouk et al., 2007 and Ramani et al., 2012). Thus, the Sdc1 exists in at least two functional states in myeloma—a cell surface receptor and a bioactive agent in the extracellular milieu—and understanding its roles in these states appear highly critical for understanding the causes of highly malignant myeloma.
As a cell surface receptor, Sdc1 has an emerging role as an organizer of integrin and growth factor receptor signaling. The best-characterized example involves the insulin-like growth factor-1 receptor (IGF-1R) and the αvβ3- or αvβ5-integrin. These receptors are captured by an active site in the extracellular domain of Sdc1 (aa 92-119 in mouse Sdc1, 93-120 in human Sdc1); their capture by Sdc1 at sites of matrix adhesion promotes activation of the IGF-1R, which generates an inside-out signal that activates the integrins (Beauvais et al., 2009; Beauvais and Rapraeger, 2010; and McQuade et al., 2006). A peptide that mimics the active site in human Sdc1 (synstatin 93-120, also called SSTNIGFIR) disrupts the assembly of this complex on tumor cells and activated vascular endothelial cells, blocks tumor growth and tumor-induced angiogenesis, and is a candidate for therapeutic intervention in human disease. These findings suggest that Sdc1, either as a cell surface receptor, or when shed from the cell surface, has a role in activating receptor tyrosine kinases and/or integrins.
VLA-4 (very late antigen-4, or the α4β1 integrin) participates in the infiltration of leucocytes and lymphocytes (Alon and Feigelson, 2002; Alon et al., 1995). In addition to using VLA-4 for extravasation from the blood stream, myeloma cells rely on VLA-4 to engage bone marrow stromal cells, and fibronectin (FN) in the marrow extracellular matrix (ECM) for growth and survival (Sanz-Rodriguez et al., 1999; Vande Broek et al., 2008) and to resist therapeutic drugs (Noborio et al., 2009) (e.g., “cell adhesion-mediated drug resistance (CAM-DR)” (Meads et al., 2008; Damiano et al., 2000; Damiano et al., 1999; Schmidmaier et al., 2006). Binding to VCAM-1 on marrow stromal cells also causes release of MIP-1α and MIP-1β, activating osteoclasts and bone erosion (Abe et al., 2009; Michigami et al., 2000).
Angiogenesis and lymphangiogenesis also play important roles in tumor growth and metastasis by providing nutrients exchange as well as avenues for tumor cell extravasation, including in hematological malignancies (Orpana and Salven, 2002). VLA-4 expression is required for angiogenesis by both vascular and lymphatic endothelial cells and its activity is especially prominent in tumors (Garmy-Susini et al., 2013; 2010; 2005). Its matrix ligand, FN, is deposited within the growing vascular and lymphatic microvessels and VCAM-1 is prominently expressed on mural cells (pericytes) that support vascular endothelial cells.
Vascular endothelial cells also rely on vascular endothelial growth factor receptor-2 (VEGFR2) and this receptor tyrosine kinase is often aberrantly expressed in many tumors as well, including in multiple myeloma (Kumar et al., 2003 and Ria et al., 2003). VEGFR2 inhibitors have been shown to block proliferation and migration of patient-derived myeloma cells (Martinelli et al., 2001). Bone marrow angiogenesis involving VEGFR2 also plays an important role in the progression of multiple myeloma as in other hematological malignancies (Rajkumar et al., 2000; Vacca et al., 1994 and Rajkumar et al., 2002). Interestingly, Sdc1 extracellular domain shed from myeloma cells expressing high levels of HPSE has been shown to promote VEGF-dependent angiogenesis in vitro. This depends on its HS chains, but also on its core protein, suggesting the presence of one or more active sites responsible for the bioactivity of the shed Sdc1.